Superplastic forming/diffusion bonding (SPF/DB) technology has emerged for the production of airframe components utilizing flat metal sheets in the fabrication process. Airframe components require only the simplest of skin thickness variations. Typically, such variations are whatever results from forming a nominally uniform thickness sheet into a particular contour. In some cases the thickness may be increased, locally, by the bonding of doublers or other details. Limited efforts have also been made to achieve different thickness level in part by chemical milling selected areas uniformly or with a taper prior to processing. However, the process is still essentially the bonding together of thin flat sheets.
The application of SPF/DB processing to produce aircraft engine components was initiated about 1976 as an extension of the airframe technology. Many aircraft gas turbine applications, for example, compressor fan blading, require that the mass distribution, that is the thickness, of the component structure vary extensively as a function of location on the part. Using prior art SPF/DB processes, that is, starting with flat rolled sheets as the processed raw material, it is possible to achieve complex mass distribution within a hollow structural component. This is done by cutting sheet details to diverse plan forms so that when properly stacked together they comprise a multi-layered, contoured aggregate that may be diffusion bonded into a monolithic mass that approximates the desired distribution of mass in the component. For example, early developmental application of SPF/DB to fan blades has been accomplished by this extension of the airframe SPF/DB technology.
A number of problems exist with applying the prior art multi-layer SPF/DB technology to making hollow components having mass distributions which vary extensively as a function of location on the part. It has been found that in order to approximate the desired mass distribution in a typical hollow fan blade as many as eighty (80) differently shaped details have to be cut from sheet stock, stacked together and consolidated. This requires large expenditures of labor and is accordingly quite expensive. Flat rolled sheet has a typical thickness tolerance of .+-.about 10%. Thus, when numerous sheet details are stacked together the aggregate thickness is difficult to control in a reproducible manner. This is of special concern in the case of rotating components, such as fan blades, where moment weight control must be at a high level. An assembly consisting of stacked sheet details provides only an approximation of the desired mass distribution. To further approach that distribution the prior art method has required that deformation of the bonded details occurs in closing of the final tooling to move mass from over-thick regions to under-thick regions. Prediction of the effectiveness of this is difficult at best whereby considerable trial and error is involved. Control of repeatability is difficult as is inspection to maximize repeatability.
The edges of sheet details, especially ones of heavier gauge, create a step type void in the as-bonded workpiece that may not completely close up as a result of the tool closing deformation. These voids represent an undesirable internal defect. This effect can be mitigated by bevelling of individual details to feather edges, but only at considerable labor costs, and with added difficulties and without complete resolution of the defect problem in a readily inspectable manner.
In the case of details that do not extend into trim areas, but terminate within the part itself, such details must be spot welded in their assembly stack position. The weld nuggets are then buried within the component leading to a questionable effect on component integrity.
Another problem also exists in superplastic forming to form hollow components having mass distributions which vary extensively as a function of location on the part. This problem is that in order for the forming gas to penetrate the stopoff areas in a diffusion bonded sandwich structure the unbonded portions of the face sheets must sequentially expand to arch slightly starting nearest the gas source. Arching of each successive section is initiated by arching of the previous adjacent section. If the thickness of the face sheets increases away from the gas source there is increasing resistance to inflation until the pressure required to provide inflation becomes so high that when opening occurs the strain rate is so high that rupture occurs from the intermediate core sheet.
The present invention is directed to overcoming one or more of the problems as set forth above.